1. Field of the Invention
The present invention generally relates to the fields of borehole geology, geomechanics and geophysics. In particular, the identification and evaluation of depth zones having subsurface deviatoric stress characteristics, i.e. the principal stress directions and stress ellipsoid shape factor R, using borehole sonic anisotropy directions in one or multiple deviated boreholes, either independently or in combination with other data.
2. Background of the Invention
Known methods and devices for extracting oil and natural gas from subterranean formations include drilling boreholes into hydrocarbon-bearing zones, building a well completion, and then recovering the product. Various sensors are utilized in order to enhance both the creation of the borehole and the productivity of the completed well. For example, wireline and logging-while-drilling sonic tools are utilized to measure the dynamic elastic properties of the formation around the borehole using compressional and shear velocity measurements. When the elastic properties of the formation are anisotropic, several velocities can be measured and used to partially or totally characterize the anisotropic elastic tensor, depending on the propagation and polarization direction. Various conditions can cause anisotropy, including but not limited to intrinsic rock properties, fractures, and non-equal principal stresses. The latter condition has some implications for wellbore stability, optimal hydraulic-fracturing, completion design, and other geophysical and petrophysical applications.
Further, having a good knowledge of existing stressed state data in a formation can be required information for planning drilling operations and mine construction. In those situations, poor estimates of effective stresses may lead to additional costs and safety problems related to geological hazards and instability of borehole or mine. Furthermore, the development of many existing oil fields and orientation of fractures are typically controlled by direction of maximum horizontal stress. Therefore, stress characterization performed prior to production may reduce risk in reservoir management decisions, particularly for production in areas.
Definition of Rock Stress
The importance of rock stress is well known for most subsurface rock activities such as in mining and oil extraction, which rely on knowledge and application of rock mechanics. The stress at a point within a rock mass is a tensor quantity that has three normal stress components acting perpendicular to the faces of a small cube, and six shear stress components acting along the faces [Hudson, J. A., F. H. Cornet, R. Christiansson, ISRM Suggested Methods for rock stress estimation Part 1: Strategy for rock stress estimation, International Journal of Rock Mechanics & Mining Sciences 40 (2003) 991998]. From moment conservation and equilibrium conditions, the nine component stress tensor has six independent components. The stress state, σ, is specified either by: (a) the three normal stresses and the three shear stresses acting on the three specified orthogonal planes determined by a set of x, y, and z-axes; or (b) the directions (defined for example by the Euler's angles) and magnitudes (for instance σ1>σ2>σ3) of the three principal stresses. Alternatively, the magnitudes of the stress tensor can be represented by the combination of the following three parameters: (i) R, the deviatoric shape factor (also called “ellipsoid” shape factor) of the stress tensor, R=(σ2−σ3)/(σ1−σ3), (ii) the ratio σ3/σ1, and (iii) the magnitude of σ1. The relative deviatoric component of the stress tensor can then be written as a tensor, σd with the same principal directions as σ and magnitudes equal to (1, R, 0).
Overview of Existing Methods for Rock Stress Estimation
Techniques to estimate subsurface rock stress have been studied extensively by the rock mechanics scientific community. For a recent review of the existing methods, refer to a series of articles entitled: “ISRM Suggested Methods for rock stress estimation” [Hudson, J. A., F. H. Cornet, R. Christiansson, ISRM Suggested Methods for rock stress estimation Part 1: Strategy for rock stress estimation, International Journal of Rock Mechanics & Mining Sciences 40 (2003) 991998; Sjoberg, J., R. Christiansson, J. A. Hudson, ISRM Suggested Methods for rock stress estimation Part 2: overcoring methods, International Journal of Rock Mechanics & Mining Sciences 40 (2003) 9991010; Haimson, B. C., F. H. Cornet, ISRM Suggested Methods for rock stress estimation Part 3: hydraulic fracturing (HF) and/or hydraulic testing of pre-existing fractures (HTPF), International Journal of Rock Mechanics & Mining Sciences 40 (2003) 10111020; R. Christiansson, J. A. Hudson, ISRM Suggested Methods for rock stress estimation Part 4: Quality control of rock stress estimation, International Journal of Rock Mechanics & Mining Sciences 40 (2003) 10211025; C. Fairhurst, Stress estimation in rock: a brief history and review, International Journal of Rock Mechanics & Mining Sciences 40 (2003) 957973] or to a recent book titled “Reservoir Geomechanics” [M. D. Zoback, Reservoir Geomechanics (2007), Cambridge University Press].
Stress is a three-dimensional quantity by nature and potentially varies spatially in all earth coordinate directions (North, East, vertical or predefined Cartesian x, y and z axes). For a complete stress tensor determination, the magnitudes and directions of the three principal stresses have to be estimated at each point in the subsurface. Most methods proceed along the following steps, for example:                (1) First step, may consider whether the vertical direction is a principal stress direction (from topography, geological evidence and other information);        (2) Second step, estimate the vertical stress component magnitude (from the rock density and overburden depth);        (3) Third step, consider indications of the principal stress directions and the ratio of stress differences from surface observations (e.g. earthquakes focal plane solutions inversion or seismic shear wave anisotropy);        (4) Fourth step, establish the minimum principal stress orientation using subsurface observations such as failure observations on borehole images (hydraulic fractures, drilling induced fractures or borehole breakout orientations) or borehole sonic anisotropy directions;        (5) Fifth step, establish the complete stress state at one or more locations from one or several of the following steps:                    (5a) from hydraulic fracturing tests in boreholes by establishing the minimum principal stress and maximum principal stress magnitude (hydraulic fracturing tests, borehole failure analysis or borehole sonic anisotropy);            (5b) using indirect methods on borehole core (such as the Kaiser effect and differential strain analysis);            (5c) from overcoring tests;            (5d) by hydraulic testing in boreholes of pre-existing fractures (HTPF); and                        (6) Sixth step, finally establish the variation of the stress state across the site due to different geological strata and fractures (as estimated through numerical analyses and further measurements).In summarizing the previous steps, the first task is most always to determine the principal directions (steps 1 to 4), and the second task is likely to determine partial or full measure of the magnitudes (steps 5 and 6), i.e. any of the possible R, σ1, σ2, σ3, and σ3/σ1. The ellipsoid shape factor of the stress tensor, R=(σ2−σ3)/(σ1−σ3) is an important quantity that can sometimes be estimated more easily from absolute stress magnitudes.        
Overview of Step 1 Above
When step 1 indicates that the vertical direction is not a principal stress direction (due to topography, complex geological strata, fractures to cite a few examples), most known methods used to find the principal stress directions would no longer apply. For example, two main classes of methods have tried to overcome this problem include: (1) the first class yields indications of the principal stress directions from earthquakes focal plane solutions inversion [Gephart, J. & Forsyth, D., 1984, a method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence, J. Geophys. Res., 89 (2), 2177-2180]. Wherein, the solutions from the above noted methods usually suppose a large volume in which the stress is homogeneous, and the validity of such solutions can still be debated; (2) the second class uses hydraulic testing of pre-existing fractures (HTPF) in boreholes. Thus, the second class obviously is not applicable if there are no preexisting fractures in the borehole [Cornet F H. The HTPF and the integrated stress determination methods, in: Hudson, editor; comprehensive rock engineering, vol. 3 Oxford: Pergamon Press; 1993; pp 413-32 (chapter 15)].
Overview of Step 2 Above
Regarding step 2 above, when the vertical direction is a principal stress direction, the vertical magnitude can be estimated provided one or several boreholes exist, and the measurement of rock bulk density can be performed using standard nuclear logging tools from the surface to the depth of interest (and then integrated over depth). This step is one of the most easily achievable, one small limitation being practical as density logs are rarely recorded up to the surface (missing data are generally extrapolated).
Overview of Step 3 Above
Regarding step 3 above, a major limitation from earthquakes focal plane solutions inversion or seismic shear wave anisotropy solutions can be coming from the fact that the data are not consistently available, especially on oilfield basins. As mentioned above, another limitation from earthquakes focal plane inversion is that the validity of the method is still debated. Seismic shear wave anisotropy suffers from the ambiguity in the interpretation of the cause of the anisotropy as being fractured-induced, intrinsic-induced or stress-induced [U.S. Pat. No. 6,714,873 B2]. At most, such methods provide indications on stress field directions but rarely yield reliable results on R (the ratio of stress differences).
Overview of Step 4 Above
Regarding step 4 above, borehole images logs have been successful at estimating the direction of the minimum or maximum horizontal principal stress from failure observations such as drilling induced fractures, borehole breakout orientations or hydraulic fractures [Luthi, S. M., 2000, Geological well logs: their use in reservoir modeling: Springer; M. D. Zoback, Reservoir Geomechanics (2007), Cambridge University Press]. Limitations include the fact that stress related failures are not always observed in a borehole (i.e. no data), and that the interpretation of stress-related failure can be misinterpreted because of the presence of natural fractures or borehole damages (i.e. false positives or negatives). Borehole sonic anisotropy techniques have been developed using dipole shear anisotropy to estimate the maximum horizontal stress direction, σH [Esmersoy, C., Kane, M., Boyd, A., and Denoo, S., 1995, Fracture and stress evaluation using dipole shear anisotropy logs: in 36th Annu. Logging Symp. Trans., SPWLA, 1-12]. One major limitation is that the cause of the observed sonic anisotropy needs to be identified as being stress-related and not fracture-induced or intrinsic-induced. Overall both image and sonic log techniques for stress direction estimation have been developed for vertical boreholes, but these techniques do not apply when boreholes are deviated more than several degrees.
Overview of Steps 5 and 6 Above
Regarding steps 5 and 6 above, one of the most frequently applied and reliable techniques to estimate the minimum (horizontal) principal stress is using hydraulic fracturing tests in boreholes. However, the magnitude of the maximum (horizontal) principal stress is more difficult to estimate. Several techniques have been developed: Using borehole breakouts identified on borehole images [Zoback M. D., Moos D., Mastin L. & Anderson R. N. (1985), Well bore breakouts and in situ stress, Journal of geophysical research, 90, B7, 5523-5530; Vernik L. & Zoback M. D. (1992), Estimation of the maximum horizontal principal stress magnitude from stress-induced well bore breakouts in the Cajon Pass scientific research borehole, Journal of Geophysical Research, 97, B4, 5109-5119], or using borehole sonic anisotropy [Sinha, B., Method for estimating formation in-situ stress magnitudes using a sonic borehole tool, U.S. Pat. No. 5,838,633 (1998); Sinha, B., Determining stress parameters of formations from multi-mode velocity data, U.S. Pat. No. 6,351,991 (2002); Sinha, B., Determination of stress characteristics of earth formations, U.S. Pat. No. 7,042,802 B2 (2006)]. Breakout techniques suffer from the lack of knowledge of failure properties. Sonic anisotropy suffers from the difficulty to identify stress-related sonic anisotropy, the presence of stress-sensitive rock formations and the requirement of the knowledge of two principal stress magnitudes and the pore pressure. Also, sonic methods are not applicable in non-vertical wells in their present form.
Overview of Identifying Causes of Anisotropy Regarding Steps 3-6 Above
As noted in Steps 3-6 above, it is important for having reliable methods for identifying the cause of the anisotropy to obtain indications on stress field directions, e.g., reliable results on R (the ratio of stress differences). For example, there are known techniques for identifying the anisotropy, such as using monopole P- and S-waves, monopole Stoneley and cross-dipole shear sonic data in the anisotropic formation to estimate one compressional and three shear moduli [Sinha, B., et al., Radial profiling of three formation shear moduli, 75th Ann. Internat. Mtg. Soc. of Expl. Geophys., 2005; U.S. Pat. No. 6,714,480, entitled “Determination of anisotropic moduli of earth formations”, to Sinha, B., et al., issued Mar. 30, 2004, incorporated by reference herein in their entireties.] An orthorhombic formation with a vertical symmetry axis is characterized by three shear moduli: c44, c55 and c66. In a vertical borehole, two vertical shear moduli (c44 and c55) can be directly estimated from azimuthal anisotropy analysis of cross-dipole waveforms. Fast-shear azimuth can be calculated using a method such as Alford rotation, and fast- and slow-shear slownesses can be estimated from the zero-frequency limits of cross-dipole dispersions [Alford, R. M., Shear data in the presence of azimuthal anisotropy, 56th Ann. Internat. Mtg., Soc. of Expl. Geophys. 1986; Esmersoy, C., et al., Dipole shear anisotropy logging, 64th Ann. Internat. Mtg, Soc. of Expl. Geophys., 1994; Sinha, B., et al., Radial profiling of three formation shear moduli, 75th Ann. Internat. Mtg. Soc. of Expl. Geophys., 2004; U.S. Pat. No. 5,214,613, entitled “Method and Apparatus for Determining Properties of Anisotropic Elastic Media” to Esmersoy, C., issued May 25, 1993; U.S. Pat. No. 5,808,963, entitled “Dipole Shear Anisotropy Logging”, to Esmersoy, C., issued Sep. 15, 1998, or for an alternative method see U.S. Pat. No. 6,718,266, entitled “Determination of dipole shear anisotropy of earth formations” to Sinha, B., et al., issued Apr. 6, 2004; Tang, X., et al, Simultaneous inversion of formation shear-wave anisotropy parameters from cross-dipole acoustic-array waveform data, Geophysics, 1999, incorporated by reference herein in their entireties]. The third shear modulus, c66, can be estimated from the Stoneley data, provided corrections are applied to remove any near-wellbore alteration and tool effects [Norris, A. N., et al., Weak elastic anisotropy and the tube wave, Geophysics, 1993, 58, 1091-1098; U.S. Pat. No. 6,714,480, entitled “Determination of anisotropic moduli of earth formations” to Sinha, B., et al., issued Mar. 30, 2004, incorporated by reference herein in their entireties]. Dipole dispersion curves are then used to identify the cause of the anisotropy of the elastic properties: (i) stress-induced effects (due to far field non equal principal stresses and near field stress concentration around the borehole) using the characteristic crossover of the dipole curves [Sinha, B. K., et al., Stress-induced azimuthal anisotropy in borehole flexural waves, Geophysics, 1996; Winkler, K. W., et al., Effects of borehole stress concentrations on dipole anisotropy measurements, Geophysics, 1998; Sinha, B. K., et al., Dipole dispersion crossover and sonic logs in a limestone reservoir, Geophysics, 2000; U.S. Pat. No. 5,398,215, entitled “Identification of Stress Induced Anisotropy in Formations” to Sinha, B., issued Mar. 14, 1995, incorporated by reference herein in their entireties], or (ii) intrinsic- or fracture-induced anisotropy using the characteristics of parallel dispersion curves [Sinha, B. K., et al., Borehole flexural modes in anisotropic formations, Geophysics, 1994; U.S. Pat. No. 5,398,215 entitled, “Identification of Stress Induced Anisotropy in Formations” to Sinha, B., issued Mar. 14, 1995, incorporated by reference herein in their entireties]. However when both fracture and stress effects are present, or when the analysis of dispersion curves is difficult to interpret due to attenuation of high frequencies [Donald, A. et al., Advancements in acoustic techniques for evaluating natural fractures, 47th Annu. Logging Symp., SPWLA, 2006, incorporated by reference herein in its entirety.], or when the symmetry axis of the anisotropic medium and the borehole axis are not aligned, the interpretation of the observed anisotropy becomes more challenging. Independent information has to be provided to confirm the observations and discriminate the relative importance of the different effects [Prioul, R., A., Donald, R., Koepsell, Z. El Marzouki, T., Bratton, 2007, Forward modeling of fracture-induced sonic anisotropy using a combination of borehole image and sonic logs, Geophysics, Vol. 72, pp. E135-E147].
Furthermore, discriminating the relative importance of the different effects can be especially important when the principal stress directions and the normal to the natural fracture planes are not aligned. The analysis of the Stoneley mode reflections and attenuation allows the identification of open fractures in the borehole, and an estimation of their apertures [U.S. Pat. No. 4,870,627, entitled “Method and apparatus for detecting and evaluating borehole wall to Hsu, K., issued Sep. 26, 1989; Homby, B. E., et al., Fracture evaluation using reflected Stoneley-wave arrivals, Geophysics; 1989; Tezuka, K., et al., Modeling of low-frequency Stoneley-wave propagation in an irregular borehole, Geophysics, 1997; U.S. Pat. No. 4,831,600, entitled, “Borehole Logging Method for Fracture Detection and Evaluation” to Hornby, B., issued May 16, 1989, incorporated by reference herein in their entireties.] In addition, the interpretation of borehole images (electrical and ultrasonic) can be used to identify either open or closed fractures [Luthi, S. M., Geological well logs: their use in reservoir modeling, Springer, 2000; U.S. Pat. No. 5,243,521, entitled, “Width determination of fractures intersecting a borehole” to Luthi, S., issued Sep. 7, 1993, incorporated by reference herein in their entireties.] It is noted fracture properties such as location and orientation can then be calculated.
A Method for Estimating Stress Directions and R from Borehole Failure (I.E. Breakouts or Tensile Fractures) in Multiple Deviated Wells and Limitations
A method using breakout or tensile fracture orientations at the borehole wall from several deviated boreholes has been developed to extract the maximum horizontal stress direction, σH, and shape factor of the stress tensor, R. [Cesaro, M., M., Gonfalini, P., Cheung, A. Etchecopar (2000), Shaping up to stress in the Apennines, Schlumberger Well Evaluation Conference, Italy 2000; Etchecopar A., et al., BorStress document and user manual, Schlumberger report, 2001] or, an alternative similar approach: [Qian, W., Pedersen, L. B. (1991), Inversion of borehole breakout orientation data, Journal of Geophysical Research, 96, B12, 20093-20107; Qian, W., Crossing, K. S., Pedersen, L. B., Dentith, M. C., List, R. D. (1994), Correct to “Inversion of borehole breakout orientation data”, Journal of Geophysical Research, 99, B1, 707-710]. Part of the method is based on work from Mastin L. (1988), Effect of borehole deviation on breakout orientations, Journal of Geophysical Research, 93, B8, 9187-9195. There are some advantages of using the above-mentioned method, which include: (1) it relies only on the orientation of the breakouts or tensile fractures and not on any failure criteria and failure properties. These criteria are difficult to estimate and can vary between several wells so not having to determine them is an important benefit of this method; (2) it is not restricted to vertical wells, and is valid using combinations of vertical and non-vertical wells or only non-verticals wells. At least two wells with different orientations are required or a single well with different well trajectory orientations in the volume of interest; and (3) from knowledge of R plus the magnitudes of vertical and minimum horizontal stresses, it is possible to estimate the maximum horizontal stress magnitude. However, there are many limitations to using the above-mentioned method, such as: (1) this method is not applicable if there is no borehole image or no breakout observed (no data); (2) no other data (e.g. sonic anisotropy) can be used in this method in its present form; (3) the vertical direction is assumed to be a principal direction; (4) at least two wells with different well orientations are required; and (5) when combining data from several wells, the hypothesis of homogeneous stress field in terms of its directions and shape factor has to be satisfied for a given volume.
Another Method for Estimating Stress Directions Estimation of the Stress Ellipsoid Shape Factor R Using Anisotropic Elastic Moduli from Sonic Anisotropy Shear Slowness Data
This method discloses an estimation of the stress ellipsoid shape factor R using anisotropic elastic moduli from sonic anisotropy shear slowness data [V. Pistre, Y. GongRui; B. Sinha, R. Prioul, Method and algorithm to determine the geo-stresses regime factor Q from borehole sonic measurement, U.S. Provisional Patent Application No. 60.000000]. However, even though this method discloses using a single borehole, the method is limited to conditions where the borehole axis and one of the principal stresses are substantially aligned (mainly in the vertical direction). Further, this method also requires the knowledge of sonic shear slowness data. In fact, this method cannot estimate principal stress directions and the stress ellipsoid shape factor R using borehole sonic anisotropy directions in one or multiple deviated boreholes, either independently or in combination with borehole failure directions (e.g. breakouts or tensile fractures) from image log data. In addition, nor can this method consider the case, when the borehole axis and the principal stresses are not aligned.
Therefore, there is a need for methods and devises that overcome the above noted limitations of the prior art. By non-limiting example, methods that can estimate subsurface principal stress directions and ellipsoid shape factor R from borehole sonic log anisotropy directions and image log failure directions, e.g., to determine subsurface deviatoric stress characteristics.